1. Field of the Invention (Technical Field)
The present invention relates to a hydrogen generating apparatus, method, and system. The present invention preferably decomposes an endothermically decomposable compound. The present invention preferably uses a hydrogen permselective separator to separate methanol decomposition products into permeate (hydrogen) and retentate (primarily CO). The retentate is then fed to a heater that burns the retentate using oxygen, preferably from air, to provide heat to the reactor and/or permselective separator.
2. Description of Related Art
Note that the following discussion refers to a number of publications and references. Discussion of such publications herein is given for more complete background of the scientific principles and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Hydrogen is used in various industrial processes and fuel cells. Most hydrogen is produced by high-temperature steam reforming of organic fuels. One example is the steam reforming of methanol, in which the methanol and water are reacted at 200-300° C. over a reforming catalyst:
This steam reforming reaction is a two-step process, the first of which is methanol decomposition:
Full completion of the reforming reaction then proceeds via the water-gas-shift (WGS) reaction:

However, if the steam reforming hydrogen generator is paired with a fuel cell, then product water from the fuel cell can be recovered, blended into the reactor feed, and used in the reforming reaction. In this case, neat methanol can be used as the stored fuel, which maximizes the fuel energy density. Another benefit of neat methanol is that it can be used in a burner to supply the heat for the endothermic reforming reaction. Typically, the amount of fuel diverted to the burner is minimized and system efficiency is maximized by the extensive use of heat exchangers to vaporize the reactants and recuperate heat. Such strategies are typically employed for transportation or stationary applications of considerable power levels. On the other hand, the extensive subsystems required to manage the water and heat greatly complicate the system, further compounding operational difficulties as well as cost and reliability.
Amongst such difficulties is the appreciable byproduct CO formed by the steam reforming reaction, which is a poison to low-temperature fuel cell systems. The CO level can be lowered to ppm levels using high steam-to-methanol ratios and additional WGS and preferential oxidation reactors, resulting in an even yet more complicated system. Alternatively, the hydrogen can be separated from the reformate using a permselective membrane such as palladium. The hydrogen permeate is then delivered to the end-use (for example, a fuel cell). The retentate, which consists primarily of CO2, as well as lesser amounts of CO, remnant H2, and excess reaction water, has a low heat value but can be used to augment the burner fuel. The reformer and separator may optionally be combined to make a “membrane reactor,” which has certain advantages under some situations. The general approach of combining fuel reforming and hydrogen separations is well known in the art. See, for example, U.S. Pat. Nos. 5,997,594 and 6,221,117.
Because of the high efficiencies that can be attained, albeit with complicated systems, steam reforming is the preferred large-scale source of hydrogen (e.g., for industrial and stationary applications). However, steam reforming is difficult to implement in small systems. Scaling down the highly integrated and complicated systems discussed above is difficult and expensive. Particularly, heat losses become much more pronounced. As device sizes decrease, the surface area per unit volume of the device increases. As such, a device measuring one-tenth as large as an otherwise equivalent version will have a tenfold greater heat loss (surface area) per unit capacity (volume) than the larger device. Thus, additional challenges and difficulties are encountered in developing small-scale thermal process systems.
Because of the heat loss difficulties with small systems, the enthalpies of the liquid forms of the reactants are used in the equations and calculations. Otherwise, using the vapor enthalpies, as is usual in the literature, assumes that there are no difficulties in recovering the waste heat necessary to vaporize the reactants, which is not the case in small systems.
An initial effort to simplify a portable reforming system is to eliminate the need for water recovery by using a methanol and water mixture for the fuel, instead of neat methanol. Typically, a 1:1 molar ratio (64 wt % methanol) is used per the stoichiometery of Equation 1, even though conversion is not optimal with the low steam content. The effective hydrogen density of the stored fuel is also lower and it does not burn easily in the heater, but the water management is greatly simplified. A possibly greater difficulty is the heat management, as discussed above. To some extent it is possible to compensate for the heat losses or maximize recovery from the scant heat sources available, but the system either becomes too inefficient or too complicated to be attractive for portable applications. A discussion of some of the challenges faced with portable reforming systems and a review of recent efforts is provided by Holladay (“Review of Developments in Portable Hydrogen Production Using Microreactor Technology,” J. D. Holladay, Y. Wang, and E. Jones, Chem. Rev., 104 4767-4790 (2004)). In short, for portable systems, a hydrogen generator is needed that requires few components and that uses a simple reaction process that also provides a source of ample heat.
Another approach to producing hydrogen from methanol is to forego the WGS reaction and use only the first step of the reforming process, that is, methanol decomposition (Equation 2). This process produces CO instead of CO2 as the primary by product. However, known decomposition systems suffer from many disadvantages, including for example the higher amount of heat required to liberate hydrogen and less hydrogen produced per mole of methanol (and thus lower efficiency) relative to steam reforming, and difficulties with a high CO concentration that impedes hydrogen permeation through permselective membranes. For example, the paper “Hydrogen generation in a Pd membrane fuel processor: assessment of methanol-based reaction systems,” M. P. Harold, B. Nair, G. Kolios, Chemical Engineering Science, 58 2551-2571 (2003), incorporated herein by reference, concludes that a methanol decomposition/membrane separation system is “infeasible”. Further, known methanol decomposition systems do not take advantage of the high heat value of the CO produced in the hydrogen generation reaction.